1. Field of the Invention
The present invention relates to power management and more particularly to power management circuits in integrated circuits and even more particularly to the management and conservation of battery power.
2. Description of Related Art
Cellular telephones, handheld computers, portable game terminals and other battery-powered devices commonly use Liquid Crystal Display (“LCD”) technology to permit users to receive and review text and graphics information. LCD are often backlit, using light emitting diodes (“LEDs”) to improve display readability while minimizing power usage. However, LEDs operate only when a sufficient voltage is available and illumination intensities are susceptible to variations in battery voltage levels. To reduce the impact of voltage variation on the illumination intensity, a current source is commonly used to drive LEDs at a selected current level.
A common approach to maintaining illumination levels employs boost regulators (also known as boost switchers) to ensure that LEDs are powered at adequate voltage levels. A drawback of this approach lies in the relatively power efficiencies of boost regulators, which typically operate between 50% and 80% efficiency.
Systems that operate without boost regulators may benefit from greater efficiency when operational, but suffer from limited battery operational time. The operational time of the battery is limited because in these systems, when voltage produced by the battery drops below a level required to turn on the display may become unreadable and the device unusable.
Pulse Width Modulation (“PWM”) LED drive methods are frequently used to control intensity of LED output in digital circuitry. PWM controls the intensity of an LED output by pulsing the LED with a variable duty cycle drive voltage such that the LED is turned on while drive voltage is applied and turned off when the voltage is removed. The drive voltage must be switched at sufficiently high frequencies to prevent detection of flicker by the human eye. The duty cycle of the drive voltage also affects the apparent intensity of the light emitted. This apparent intensity may be considered an average intensity over a selected period of time. In a digital circuit the average intensity may be calculated as:
            L      AVG        =                  (                              t            on                                              t              on                        +                          t              off                                      )            ×              L        MAX              ,                where ton is the duration of time when the voltage is applied to the LED in one cycle and toff is the time the voltage is turned off in one cycle; LAVG is the average luminosity of the LED and LMAX is the maximum luminosity of the LED as observed when voltage is applied to the LED.Thus, the PWM technique provides a light source of variable intensity where intensity may be controlled by increasing the pulse width of the voltage on time relative to the overall cycle time. PWM provides an alternative method to incrementally increasing or decreasing the level of an unswitched drive voltage. It should be apparent that in order to obtain an apparent intensity equal to a constant voltage driven LED, PWM suffers from the drawback that higher voltages and currents must be used, albeit for relatively short durations. This requirement for higher voltages necessitates the use of a higher voltage battery or the addition of a relatively inefficient voltage booster.        